The invention relates to an apparatus for reproducing video signals at a speed which deviates from a nominal speed, which video signals have been recorded on a record carrier in the form of a tape in mutually parallel tracks which are inclined relative to the longitudinal axis of the record carrier, which apparatus comprises tracking-error detection means for detecting the instantaneous deviation of a read head relative to the center of a track to be followed, a reference-signal generator for generating reference signals for the tracking-error detection means in order to generate a tracking-error signal which is a measure of the deviation of the read head relative to the center of the track corresponding to the generated reference signal, said reference signals being distinguishable within cycles of at least three tracks, and a tape-transport control loop for controlling the tape transport as a function of said tracking-error signal.
Such an apparatus is inter alia known from United States Patent Specification no. 4,297,733 herewith incorporated by reference. The known apparatus employs long wave track-sensing signals recorded in the tracks, the head reading the track-sensing signals produced by crosstalk from two adjacent tracks during the read-out of a specific track. The reference signals are mixing signals which, upon being mixed with the signals read by the head and containing the two crosstalk signals, result in mixing products which are filtered out to form a tracking-error signal. These mixing signals change with each track being read in a cycle of four. Other methods are also known, as, for example, consecutive tracks may be distinguished for the purpose of deriving a tracking-error signal by recording track-sensing signals at different locations in consecutive tracks. The reference signals are then time-reference signals for defining time windows by means of which said track-sensing signals can be identified and read. Alternatively, phase-reference signals may be used, or combinations of the said signal types.
During playback at the nominal tape speed, the tracking signal is utilized in order to ensure that the head follows the track. This may be effected by controlling the head position by means of actuators or by controlling the tape-transport speed.
During playback at a speed which deviates from the nominal tape speed, tracks are read several times (slow moition or still picture) or tracks are skipped (fast motion). For this purpose the reference signals are generated in a specific pattern from which reference signals may be repeated in the case of slow motion production and reference signals may be skipped in the case of fast motion. When actuators are used, the head always follows the track corresponding to the instantaneously generated reference signal. If the method of controlling the tape-transport speed is used, the head will not follow the track exactly, because such a control system is much too slow for this, but scans the tracks obliquely. However, the control system does ensure that on the average the head follows the track which corresponds to the generated tracking signal.
When actuators are used any deviating tape transport speed is possible, in principle, without problems. The actuators ensure a correct tracking regardless of the tape-transport speed. When a tape-transport speed control system is used for tracking control, only a limited number of speeds which deviate from the nominal tape-transport speed are possible. For example, playback with a speed which is two times as slow or two times as fast is possible by repeating each reference signal twice or by skipping every second reference signal. A limited number of multiples or submultiples of the nominal transport speed are also possible. As an example, playback with a speed which is two and a half times as slow is possible by alternately repeating the reference signals two times and three times.